Pressure sensor for detecting the pressure in the combustion chamber of internal-combustion engines

ABSTRACT

A pressure sensor for detecting pressure in a combustion chamber of internal combustion engines has a housing, a sensor element composed of a piezoresistive material and arranged in the housing, a diaphragm, a punch introducing a pressure to be determined onto the sensor element and located between the diaphragm and the sensor element. The punch has a counterbearing, a hybrid has a base, and a preprocessing circuit has electronic components. The sensor element, the hybrid with its base and the electronic components of the preprocessing circuit are located on the counterbearing of the punch.

The present invention relates to a pressure sensor.

More particularly, the present invention relates to a pressure sensor inwhich a sensor element is composed of a piezoresistive material andarranged in a housing. In the case of such a pressure sensor, known fromGerman Offenlegungsschrift 31 25 640.6, the piezoresistive measuringelements, such as for example thick-film resistors of cement, contactiveplastic or platinum, are printed on a base. The resistor element and thebase are arranged as close as possible to the pressure chamber in orderto be able to determine the pressure prevailing there. Furthermore, themeasuring signal is passed with the aid of electric leads to anelectronic evaluation circuit arranged outside the housing of thepressure sensor. As a result, the piezoresistive elements and theelectronic components have to be elaborately connected to one anotherwith the aid of insulated pieces of wire. Since the piezoresistivemeasuring element is exposed directly to the pressure, it is alsoexposed to the high temperatures prevailing in the combustion chamber.The flames spread there at a temperature of about 2000° C., wherebystresses can occur in the housing As a result, the pressure signal isfalsified by the high temperatures.

Furthermore, European Preliminary Published Specification 85 111 895.0discloses a pressure sensor in which the thick-film resistor is arrangedon the bottom of a base. However, this pressure sensor is intended onlyfor determining the pressure in distributor pumps. The high temperaturesprevailing in the combustion chamber would falsify the measuring signalin the case of this design as well.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apressure sensor for detecting pressure in a combustion chamber ofinternal combustion engines which avoids the disadvantages of the priorart.

In keeping with these objects and with others which will become apparenthereinafter, one feature of the present invention resides, brieflystated, in a pressure sensor in which between a diaphragm and a sensorelement there is arranged a punch which introduces a pressure to bedetermined onto the sensor element, and the sensor element and a hybridwith its base and the electronic components of the preprocessing circuitarranged thereupon are located on a counterbearing of the punch.

When the pressure sensor is designed in accordance with the presentinvention, it has the advantage that the piezoresistive measuringelement is relatively far away from the combustion chamber and the hightemperatures prevailing there, so that the measuring signal is notfalsified by the temperatures prevailing in the combustion chamber. Dueto the arrangement of the piezoresistive measuring element and thehybrid on the counterbearing of the punch, standard bonding techniquestried in practice can be used for connecting the electronic componentsand the piezoelectric elements. The various materials adapted to therespective printing techniques can be used as material for the base ofthe hybrid, which can also be used at the same time in an advantageousway as base for the thick-film resistors and semiconductor components.The hybrid can be printed in one operation simultaneously with theconductor tracks and the contact surfaces for the piezoresistive effectThe entire hybrid, including the electronic components for signalpreprocessing and the piezoresistive elements, can be checked beforefitting into the sensor housing As a result, simple assembly of thepressure sensor is possible If the piezoresistive measuring element andthe electronic components are on a common base, and if these componentsare arranged on the side of the counterbearing facing the diaphragm, itsoverall length can be reduced The hybrid can also be fitted into thehousing of the pressure sensor in the axial direction of the latterwithout elaborate design modifications. If, for example, a flexibleKapton film is used, the piezoresistive measuring elements andelectronic components can likewise be printed on in a single operation.Furthermore, this type of design produces a particularly narrow form ofthe pressure sensor. The pressure sensor can be produced inexpensively.On account of the spatial arrangement, in particular due to thearrangement on the same hybrid in direct proximity to one another,virtually the same temperature prevails with respect to the electroniccomponents and with respect to the piezoresistive elements. As a result,it is possible to dispense with elaborate measures for temperaturecompensation. A functional trimming of the piezoresistive measuringelements can be effected by simple conventional laser scoring or lasertrimming of the resistors. With the aid of electronic components it ispossible to allow a compensation of the systematic temperature responseof the piezoelectric measuring elements. In addition, it is possible forthe electrical terminal of the measuring elements not to be connected tothe housing leads, whereby a potential-free design of the sensor ispossible and consequently the measuring signal is transmitted.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a pressure sensor,

FIG. 2, FIG. 3a, FIG. 3b each show a detail and

FIG. 4 to FIG. 7 each show a modification of the exemplary embodimentaccording to FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

In FIG. 1, 10 denotes the housing of a pressure sensor 11 fordetermining the pressure in the combustion chamber of aninternal-combustion engine. It has a central, continuous stepped bore12. The opening 13 of the housing 10 facing the combustion chamber isclosed off by a diaphragm 14. The diaphragm 14 is designed as aso-called cap diaphragm. The edge of the diaphragm 14 is bent around andpushed over the end of the shank 15 of the housing 10. As a result, thediaphragm 14 sits firmly on the housing 10 but, in order to ensure amobility of the diaphragm 14, does not bear directly against the endface 16 of the shank 15. As a result, the bending region of thediaphragm 14 can move freely The diaphragm 14 is welded onto the shank15 in the region of the edge. The diaphragm 14 is formed particularlyadvantageously from a superalloy, that is to say from an alloy of, forexample, 50% Ni, 20% Cr, 20% Fe. Against the middle area of thediaphragm 14 there bears with its one end a punch 18, which with itsother end bears against a piezoresistive measuring element 19.Piezoresistive measuring elements are to be understood as elements whichchange their resistance value under the effect of pressure. For this,thick-film resistors may be used for example. As materials for this,cement, contactive plastic or platinum etc. can be used. The measuringelement 19 is printed onto the base 20 of a hybrid 21. A hybrid isnormally to be understood as a base having printed-on circuit parts,such as for example resistors etc., conductor tracks and an IC(integrated circuit), which is placed on the base and is bonded to thecircuit parts, for example by bonding wires. The punch 18 itself mayconsist of glass ceramic, in order thereby to ensure a good thermalinsulation between the diaphragm, that is to say between the pressurechamber of which the pressure is to be determined, and thepiezoresistive measuring element. The end of the punch 18 facing thehybrid 21 may be conically designed, so that its end has approximatelythe diameter of the measuring element 19. This makes it possible toguide the punch 18 in the bore 12, but on the other hand to limit thearea of pressure transfer to the size of the measuring element 19. Thebase 20 of the hybrid bears furthermore against a counterbearing 22,pressed into the bore 12.

The hybrid 21 and the end of the punch 18 are shown in more detail inFIGS. 2, 3a and 3b. The base 20 consists of an Al₂ O₃ substrate layer,on which a first contact layer 23 is printed. In a following printingoperation, the piezoresistive film 24 and subsequently the secondconductor track 25 are printed onto the first conductor track 23. Theend of the punch 18 is fastened on the second conductor track 25 withthe aid of a glass-like mass 26, in order thereby to permit a uniformforce introduction onto the resistor element 24 and to fix the punch.Furthermore, the electronic components 28, such as for exampleresistors, transistors etc., are arranged on the base 20 of the hybrid21. The electronic semiconductor components 28 and the conductor tracksare bonded with the aid of bonding wires 31. The output of thepreprocessing electronics is connected with the aid of a lead 32 to anevaluation circuit and control device (not shown) of theinternal-combustion engine, which are outside the sensor. For this, inthe counterbearing 22 there is formed a continuous bore 33, which runsaxially parallel to the bore 12 and in which the outgoing lead 32 isguided. The lead 32 is fastened in a grommet 34 of the cover 35 closingoff the bore 12. Instead of a bore 33, a segment may also be cut out onthe counterbearing 22 for guiding through the lead 32.

For protection against harmful environmental influences, such as forexample moisture, the bore 12 is filled in the region of the electroniccomponents 28 and/or in the region between the counterbearing 22 and thecover 35 with a casting compound 36.

In FIG. 2, the resistors 24 are printed in the direction of the current,for which a very high-impedance resistive paste should be used. As aresult, a greater measuring signal is obtained with an introduced forceof the same magnitude in comparison with conventional arrangements.

In the design according to FIG. 3a, the resistors 24a are printedtransversely to the direction of the current on the base 30 of Al₂ O₃substrate of the hybrid 21. In a modification of the design according toFIG. 2, in FIG. 3a the first conductor track 23a and the secondconductor track 25a are printed onto the base 20. The resistive film 24ais printed on between the two ends of the conductor tracks 23a, 25a. Therespective ends of the resistive film and of the first conductor track23a or the second conductor track 25a overlap in each case. However,this area of overlap lies outside the force-transforcing region of thepunch 18. As can be seen from FIG. 3b, it is also possible to print aplurality of resistors onto the base as part of a so-called Wheatstonebridge circuit. In this arrangement, in each case two opposite resistorsof the bridge circuit should be located in the force-introducing regionof the punch 18, whereas the two other resistors are to be printed on inthe direct vicinity of the resistors but in a region in which the forceintroduced by the punch 18 is not effective. The resistors 24a, 24bprinted on in the region of force introduction as well as the tworesistors in the vicinity are to be realised using the same technology,so that an approximately identical temperature behaviour as well as along-term stability of the resistors is obtained. This form ofarrangement of the resistors can be realised particularly simply by thethick-film technique. Due to the good temperature behaviour, it is alsopossible to dispense with an elaborate a.c. voltage amplifier aspreprocessing electronics. As a result, only a d.c. voltage amplifier,comprising just a few resistors and an operational amplifier, need bearranged on the base of the hybrid. Since only a few components arenecessary for the preprocessing circuit, the hybrid is relatively smallin construction and consequently the cross-section of the pressuresensor can also be kept relatively small. This design consequentlyresults in a combustion chamber sensor of low overall volume which is ofa relatively simple and inexpensive construction.

Instead of leading off the measuring signal from the edge of the hybridwith the aid of the lead 32 and the bore 33, it is also possible toprovide bores 40 in the base 20b of the hybrid 21. For this, bores 41are also to be formed in the counterbearing 22. Then the lead 32 is ledthrough these bores 40, 41 to the electric components 28 of the hybrid21 or their conductor tracks. For this, conductor tracks of the hybridare advantageously led through the bores 40 and contacted with the lead32 on the underside of the base 20b.

The designs according to FIGS. 5 to 7 represent special designs of thebase 20 of the hybrid 21 and the arrangement of the electroniccomponents 28 of the hybrid 21. In FIG. 5, the electronic components 28of the hybrid 21 are applied here directly on the metalliccounterbearing 22b of metallic material As material of thecounterbearing SiCrAl may be used for example. For this, thecounterbearing 22b is electrically insulated on the side facing thepunch 18 by a glass layer 43, applied by the thick-film technique.Subsequently, the components 28 and the piezoresistive measuring element19 are applied, as in FIG. 1. In the case of this embodiment, it ispossible to do without the adhesive bond between the base 20 of Al₂ O₃substrate and the counterbearing 22.

In the exemplary embodiments according to FIG. 6 and FIG. 7, the hybrid21 is arranged in the axial direction of the pressure sensor 11.Although this makes the pressure sensor 11 longer in the axialdirection, a particularly narrow construction of the pressure sensor 11is possible. As a result, the pressure sensor 11 can be adapted in aparticularly simple way to special installation situations in theinternal-combustion engine. As distinct from the previous exemplaryembodiments, the hybrid 21 is just tilted through 90°. For this, thecounterbearing is no longer cylindrically designed, but must have on oneside surface a flat face 44 on which the hybrid is applied The base ofthe hybrid is then applied on this side and also on the end face of thecounterbearing 22c facing the punch 18. In this case, it is againpossible to use a glass layer as the material for the base. However, asin FIG. 7, a flexible Kapton film 45 may also be used as base for thehybrid 21. Kapton films are commercially available and have thefollowing properties: high temperature resistance, good dielectricstrength and high flexibility.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in apressure sensor for detecting the pressure in the combustion chamber ofinternal-combustion engines, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of the invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A pressure sensor for detectingpressure in a combustion chamber of internal combustion engines,comprising a housing having an opening; a sensor element composed of apiezo-resistive material and arranged in said housing; a diaphragmfixedly mounted on said housing in the region of said opening of saidhousing so as to close said opening; a punch introducing a pressure tobe determined onto said sensor element and located in said housingbetween said diaphragm and said sensor element, said punch having an endwhich bears against an inner surface of said diaphragm, said punch alsohaving a counterbearing; a hybrid having a base; and a preprocessingcircuit having electronic components, said sensor element, said hybridwith its base and said electronic components of said preprocessingcircuit being located on said counterbearing of said punch.
 2. Apressure sensor as defined in claim 1, wherein said sensor element isarranged on said base of said hybrid.
 3. A pressure sensor as defined inclaim 1, wherein said counterbearing has a side facing said punch, saidhybrid being arranged on said side of said counterbearing.
 4. A pressuresensor as defined in claim 1, wherein said hybrid is applied in an axialdirection of said pressure sensor on a surface of said counterbearing.5. A pressure sensor as defined in claim 1, wherein said base of saidhybrid is composed of Al₂ O₃.
 6. A pressure sensor as defined in claim1, wherein said base of said hybrid is composed of a Kapton film.
 7. Apressure sensor as defined in claim 1, wherein said base of said hybridis composed of SiCrAl.
 8. A pressure sensor as defined in claim 1; andfurther comprising an insulation layer located between said base of saidhybrid and said components.
 9. A pressure sensor as defined in claim 8,wherein said hybrid is composed of SiCrAl, while said insulation layeris composed of glass.
 10. A pressure sensor as defined in claim 1,wherein said counterbearing is composed of SiCrAl and serves as saidbase of said hybrid.
 11. A pressure sensor as defined in claim 1; andfurther comprising means for bonding said electronic components.
 12. Apressure sensor as defined in claim 11, wherein said bonding meansinclude bonding wires and thick-film conductor tracks.
 13. A pressuresensor as defined in claim 1, wherein said piezoresistive element hasresistive films which are printed in direction of a current flowingthrough said resistive films.
 14. A pressure sensor as defined in claim1, wherein said punch is composed of glass ceramic.
 15. A pressuresensor as defined in claim 1, wherein said counterbearing is pressedinto said bearing.